Assessed risk for prey should depend not only on distance from a predator, but also on probability that it has been detected. If an indirectly approaching predator close enough to pose a threat suddenly turns toward prey, risk increases because the joint probability that the predator has detected and will attack is raised abruptly. I conducted field experiments on the effect of a predator turning toward prey on escape behaviour by simulating a predator, slowly approaching adult keeled earless lizards, Holbrookia propinqua, on linear paths that did not lead directly toward them, but bypassed them by various distances. At the minimum bypass distance for a path, I stopped and turned directly toward a lizard or directly away. The effect of turning varied dramatically with bypass distance. Regardless of tum direction, almost all lizards fled for very close approaches and no lizards fled for sufficiently large bypass distances. At intermediate distances a greater proportion of prey fled when I turned toward them than away from them. Possible cues used by lizards to assess risk are direction of turning per se and increased visibility of eye and facial features or other morphological correlates of orientation. Ydenberg & Dill's (1986) optimality model of escape behaviour can account for the results if prey can continually assess rapidly changing risk. By turning toward prey, an indirectly approaching predator may suddenly shift from outside to within the break-even point for risk and cost of escape, thereby eliciting flight.